1. Field of the Invention
The present invention relates to semiconductor devices having a light emitting element and a light receiving element and, more particularly, to a semiconductor device having a monolithic structure which is an integrated circuit comprising a photo-coupler or photoelectromotive force element.
2. Related Arts
It has recently become popular to provide electrical insulation using a photocoupler as a technique for isolating I/O interface portions and power supply portions of various electronic apparatuses (for example, refer to pp. 63-74 of "How to Use Opto-Electronic Devices" published by Daily Industrial News). For programmable controllers for factory automation, a number of photocouplers are used for purposes including parallel transmission of data of 16 to 64 bits and transmission of information such as abnormalities at the actuating end and the like to the controlled end. Such a photocoupler is a hybrid IC which is obtained by integrating a chip of a light emitting element utilizing a compound semiconductor such as GaAs and a chip of a light receiving element utilizing Si into a single package. A great number of such photocouplers are arranged on a substrate to be used. Japanese unexamined patent publication No. (SHO) 48-46278 corresponding to U.S. Pat. No. 3,914,137 to Huffman et al. discloses a conventional technique for integrating such photocouplers on a single chip to be more compact as a whole.
However, when this technique is used for forming a plurality of pairs of light emitting and receiving elements (channels) on the same substrate, a problem arises in that crosstalk occurs between optical signals over different channels. The term "crosstalk" used herein refers to a phenomenon that optical signals leak to and propagate in adjoining elements. The photocoupler disclosed in Japanese unexamined patent publication No. (SHO) 48-46278 uses a GaAs light emitting diode formed on the surface of a silicon substrate as a light emitting element. Since the refractive indices of Si and GaAs against light having a wavelength of about 1 .mu.m are substantially equal to each other, i.e., they are both on the order of 3.5, the light emitted by the GaAs light emitting diode enters not only the waveguide but also the silicon substrate. The light propagates to the light receiving elements of the adjoining channels and optical waveguides, causing crosstalk between optical signals. As a result, the signals cannot be transmitted properly.